Arrange the following ligands in increasing order of their field strength:
(A) NH3
(B) CO
(C) NCS−
(D) CN−
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (C), (A), (D), (B)
The field strength of a ligand is its ability to split the d orbitals of the metal (the crystal field splitting, Δo). Ligands arranged in increasing order of field strength form the spectrochemical series, which is found experimentally from the absorption spectra of complexes. In it, \( \ce{NCS^{-}} \) comes just after \( \ce{H2O} \), \( \ce{NH3} \) comes later, and \( \ce{CN^{-}} \) and CO are at the strong end:
(C) \( \ce{NCS^{-}} \) < (A) \( \ce{NH3} \) < (D) \( \ce{CN^{-}} \) < (B) CO
Shortcut: \( \ce{NCS^{-}} \) (C) is the weakest of the four ligands. Only Option (2) starts with (C).
NCERT (2026-27 edition):
Crystal Field Theory – …
